1. Field of the Invention
The invention relates to low frequency seismic/acoustic transducers in general and to musical instruments and line sensors in particular.
2. Description of the Prior Art
There are a variety of methods known in the prior art for producing a modulated electrical signal proportional to a disturbance. Such techniques include metal "electronic" strings, inductive pick-ups or devices which sense the change in the electronic parameters of a system such as a variation in capacitance. Such devices have problems both in the musical field as well as in the security field. When steel strings are used on musical instruments they usually sound tinny or metallic in pitch. This may produce an "electric sound" wherein the instrument from which the sound issues is secondary in effect to the metallic sound of the strings. Metal strings or sensors as used in the security field loose their effectiveness of their locations are known. Unfortunately, metal sensors may be detected with simple metal detectors as currently available. By employing fiberoptics in both the musical and security fields there are certain unique advantages obtained not the least of which is cost. In the application of musical instruments the sound detected is more truly representative of the instrument itself due to the flexibility of silicon. If fiberoptics as taught by this invention are applied to the security field they would be virtually undetectable due to their non-metallic nature.
Where fibers are used in the prior art as acoustic transducers they have generally not lent themselves well to either musical or security applications. Such techniques are discussed in U.S. Pat. No. 3,920,982 issued to Harris and in U.S. Pat. No. 4,086,484 issued to Steensma. According to the disclosure a phonon-photon interaction produces a change in the index of refraction of the fiber thus allowing optical energy to escape from the fiber which is modulated by the frequency of the acoustic wave. While the method works well for multi-plexing broad band information onto a continuous optical carrier it does not work well in the low frequency range, typically less than 20 kilocycles such as required by either a security sensor or in the context of a musical instrument string. The photon-phonon interaction depends upon the ratio of the velocity of sound and light in the medium which are typically in the order of one to one million. Thus a photon which is 10.sup.5 times more energetic than a phonon can be controlled by a phonon, requring for modulation an acoustic wave in the ultrasonic region far beyond the sound of a footstep or musical note.
Other acousto-optic transducers such as described in U.S. Pat. No. 4,071,753 require that the fiber be broken along its length and that the optical coupling coefficient (a measure of the alignment of two fibers) be modulated by the vibration of the joint. In an effective musical instrument string it is necessary for this fiber to remain taught and vibrate around a resonate frequency determined by the length, density of the material and tension in the fiber. Clearly any break in the fiber would effect the harmonic qualities that are desired. Likewise in an effective security line type sensor it is desirable for the transducer to be active throughout its entire length. A dead section could represent a weakness and would not be tolerable. A line sensor operating on the principle of a modulated coupling coefficient would require far too many breaks over the desired 100 meter range and therefore severely limit the total amount of light received by the detector.
The present invention contemplates the use of a laser and an optical fiber but does not rely on a photon-phonon interaction nor on a modulated coupling coefficient to produce a modulation in the output power. Instead, the disclosed invention requires a mechanical disturbance of the fiber such as that caused by the resonance of the fiber under tension as might be found on a musical instrument or as caused by its vibration in sympathy with the surrounding ground as might be found in response to an intruder's footstep. The coherent optical signal when transmitted along the fiber breaks up into a large number of individual rays. Each ray seeks its own path along the fiber. For example, out of a bundle of rays there might be two, A and B respectively. The relative path length between ray A and ray B produces an interference pattern in the output of the fiber. As the fiber is disturbed the interference pattern changes due to the change in the relative path length of the rays as they travel the length of the fiber. A square law power detector is used to detect this change in the interference pattern. This change is detected as a modulation of the output power due to the wave nature of the light as discussed by A. McLean Nicholson in U.S. Pat. No. 1,951,523.